LITTLE KNOWN FACTS ABOUT BIREFRINGENT CRYSTAL.

各向异性透明晶体如方解石、石英等的折射率，是其固有的特性，称为永久双折射。For optical fibers together with other waveguides, it is a lot more suitable to take into account the difference of effective refractive indices. That is specifically related to the main difference in imaginary values of your propagation c